Heads up display technology provides in the field of view of an individual, such as a pilot, a display of information, such as words, symbols, images, pictures, etc., while the pilot is viewing something else, such as the scene or view outside of an airplane. An advantage of a heads up display is that the pilot, or other individual using such heads up display, does not have to move his head to change the point of focus of his eyes, i.e., the main image being viewed by his eyes. In some instances the pilot may not even have to move his eyes while changing his point of focus.
Typically in a heads up display an image that contains specific information is projected ahead (forward) of the eyes of the viewer. If the viewer is wearing goggles, the goggles should not interfere with the projected image. Also, if the viewer is wearing a visor over a helmet or as part of a helmet, such visor should not interfere with the projected image. In some instances the visor is used both as a protective layer to block bright sunlight, for example, from the eyes of the viewer and also as a screen onto which the heads up display information can be projected for viewing by the viewer. Such projection may be by specialized cathode ray tube (CRT) projection apparatus, by holographic projection apparatus, or by other apparatus, which are known systems.
Various devices have been employed to protect the eyes from injury due to high intensity electromagnetic energy. One example is the liquid crystal apparatus disclosed in U.S. Pat. No. 4,765,719 by James L. Fergason. In the past other protection devices have been used in the goggles worn by a pilot, for example, to control and/or to minimize the intensity of and/or to block the electromagnetic energy transmitted toward the eyes of the pilot. A disadvantage with using eye protection in goggles in the past has been that the heads up display information which is projected outside of the goggles also is reduced in intensity or may even be blocked entirely from the pilot's eyes. Consideration has been given in the past to using means in the visor to provide eye protection from high intensity electromagnetic energy. Such a system would provide a variation in control of the transmitted light while still permitting the heads up display to be viewed through goggles from within the visor. However, the difficulty, complexity and cost of manufacturing such a large visor with eye protection light controlling characteristics are substantial, and reduced quality of viewing clarity through a complex visor system is envisioned.
Liquid crystal devices for controlling the transmission of light therethrough are known. One example is a twisted nematic liquid crystal apparatus. The twisted nematic liquid crystal apparatus includes a twisted nematic liquid crystal cell positioned between a pair of plane (linear) polarizers and selectively operable to rotate or not to rotate the plane of polarization of plane polarized light incident on the cell, e.g., as a function of whether or not a prescribed input, such as an electric field, is applied across the liquid crystal cell. Sometimes the twisted nematic liquid crystal cell is referred to as being operative to provide a wave guiding effect as light travels therethrough. If the two polarizers on the opposite sides of a twisted nematic liquid crystal cell are crossed, then light will be transmitted through such apparatus when the liquid crystal cell rotates the plane of polarization by ninety degrees or light will be blocked when the cell does not rotate the plane of polarization. Conversely, if such polarizers are parallel, then light will be blocked when the cell rotates the plane of polarization by ninety degrees and will be transmitted through the apparatus when the cell is not rotating the plane of polarization.
Other liquid crystal devices useful in controlling transmission of light are birefringent liquid crystal devices and optical retarders. An exemplary birefringent liquid crystal device, which also is or functions as a variable optical retarder, includes a liquid crystal cell that operates on the principle of birefringence and/or retardation to rotate or not to rotate the plane of polarization of incident polarized light. Such a birefringent liquid crystal device or optical retarder typically is positioned between a pair of polarizers, usually plane polarizers. The resulting optical effect, i.e., either blocking or transmitting of light, is a function of whether the polarizers are parallel or crossed and whether or not or the degree that the birefringent liquid crystal cell or optical retarder is rotating the plane of polarized light incident thereon. An example of a birefringent liquid crystal cell that provides variable optical retardation sometimes is referred to as a surface mode liquid crystal cell or device and is disclosed in U.S. Pat. Nos. 4,385,806, 4,540,243 and Re 32,521 by James L. Fergason. A birefringent liquid crystal cell/variable optical retarder also is shown in U.S. Pat. Nos. 4,566,758, 4,582,396 and 4,719,507. The speed of response of a birefringent liquid crystal device or variable optical retarder device, e.g., of the type disclosed in the mentioned patents, typically is faster than the usual speed of response for twisted nematic liquid crystal cells.